<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    
    .container {
      width: 100px;
      height: 100px;
      border: 1px solid black;

    }
    .left, .right {
      display: inline;
      width: 50px;
      
    }
    .left {
      background-color: red;

    }
    .right {
      background-color: green;
    }
  </style>
</head>
<body>
  <div class="container">
    <div class="left">left555</div><div class="right">right</div>
  </div>
</body>
</html>